Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Deferred Compensation [Member]	Accumulated Other Comprehensive Earnings (Losses) [Member]
Beginning Balance at Dec. 31, 2018	$ 1,053,295	$ 678	$ 443,114	$ 606,658	$ (7,507)	$ 7,507	$ 2,845
Beginning Balance, Shares at Dec. 31, 2018		67,753,133
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2018					(467,811)
Net earnings (unaudited)	80,345			80,345
Stock option exercises (unaudited)	3,186	$ 2	3,184
Stock option exercises, Shares		168,349
Restricted stock grant (unaudited)	1,426		1,426
Restricted stock grant (unaudited), Shares		47,532
Cash dividends declared	(30,531)			(30,531)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	56,402						56,402
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (316)	316
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					2,709
Stock option expense (unaudited)	625		625
Two-for-one stock spllit in the form of a 100% stock dividend (unaudited)		$ 678		(678)
Two-for-one stock spllit in the form of a 100% stock dividend (unaudited), shares		67,840,210			(464,339)
Ending Balance at Jun. 30, 2019	$ 1,164,748	$ 1,358	448,349	655,794	$ (7,823)	7,823	59,247
Ending Balance, Shares at Jun. 30, 2019		135,809,224
Treasury Stock, Ending Balance, Shares at Jun. 30, 2019	(929,441)				(929,441)
Beginning Balance at Mar. 31, 2019	$ 1,107,522	$ 1,356	445,672	629,988	$ (7,660)	7,660	30,506
Beginning Balance, Shares at Mar. 31, 2019		135,680,420
Treasury Stock, Beginning Balance, Shares at Mar. 31, 2019					(928,678)
Net earnings (unaudited)	42,093			42,093
Stock option exercises (unaudited)	940	$ 2	938
Stock option exercises, Shares		81,272
Restricted stock grant (unaudited)	1,426		1,426
Restricted stock grant (unaudited), Shares		47,532
Cash dividends declared	(16,287)			(16,287)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	28,741						28,741
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (163)	163
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					(763)
Stock option expense (unaudited)	313		313
Ending Balance at Jun. 30, 2019	$ 1,164,748	$ 1,358	448,349	655,794	$ (7,823)	7,823	59,247
Ending Balance, Shares at Jun. 30, 2019		135,809,224
Treasury Stock, Ending Balance, Shares at Jun. 30, 2019	(929,441)				(929,441)
Beginning Balance at Dec. 31, 2019	$ 1,227,197	$ 1,359	450,676	707,656	$ (8,222)	8,222	67,506
Beginning Balance, Shares at Dec. 31, 2019		135,891,755
Stock issued in acquisition of Commercial Bancshares, Inc.	$ 220,273	$ 63	220,210
Stock issued in acquisition of Commercial Bancshares, Inc, Shares		6,275,574
Treasury Stock, Beginning Balance, Shares at Dec. 31, 2019	(927,408)				(927,408)
Net earnings (unaudited)	$ 90,701			90,701
Stock option exercises (unaudited)	2,481	$ 1	2,480
Stock option exercises, Shares		160,720
Restricted stock grant (unaudited)	913		913
Restricted stock grant (unaudited), Shares		32,149
Cash dividends declared	(35,527)			(35,527)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	83,730						83,730
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (475)	475
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					(4,610)
Stock option expense (unaudited)	689		689
Shares repurchased under stock repurchase authorization (unaudited)	(8,008)	$ (3)	(8,005)
Shares repurchased under stock repurchase authorization, shares		(324,802)
Ending Balance at Jun. 30, 2020	$ 1,582,449	$ 1,420	666,963	762,830	$ (8,697)	8,697	151,236
Ending Balance, Shares at Jun. 30, 2020		142,035,396
Treasury Stock, Ending Balance, Shares at Jun. 30, 2020	(932,018)				(932,018)
Beginning Balance at Mar. 31, 2020	$ 1,526,362	$ 1,423	673,535	727,828	$ (8,437)	8,437	123,576
Beginning Balance, Shares at Mar. 31, 2020		142,314,930
Treasury Stock, Beginning Balance, Shares at Mar. 31, 2020					(928,417)
Net earnings (unaudited)	53,470	$ 0		53,470
Stock option exercises (unaudited)	289		289
Stock option exercises, Shares		16,532
Restricted stock grant (unaudited)	795		795
Restricted stock grant (unaudited), Shares		28,736
Cash dividends declared	(18,468)			(18,468)
Change in unrealized gain in investment securities available-for-sale, net of related income taxes (unaudited)	27,660						27,660
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net (unaudited)					$ (260)	260
Shares purchased (redeemed) in connection with directors' deferred compensation plan, net, Shares					(3,601)
Stock option expense (unaudited)	349		349
Shares repurchased under stock repurchase authorization (unaudited)	(8,008)	$ (3)	(8,005)
Shares repurchased under stock repurchase authorization, shares		(324,802)
Ending Balance at Jun. 30, 2020	$ 1,582,449	$ 1,420	$ 666,963	$ 762,830	$ (8,697)	$ 8,697	$ 151,236
Ending Balance, Shares at Jun. 30, 2020		142,035,396
Treasury Stock, Ending Balance, Shares at Jun. 30, 2020	(932,018)				(932,018)